Fix tungstenite buffering parameters for uart WS

When tungstenite was updated to 0.20.x, the structure of the buffering
parameters exposed by its websocket configuration changed significantly.
The parameters chosen can cause `WriteBufferFull` errors if enough
output accumulates in the history buffer to be sent in a message which
exceeds the unnecessarily small `max_write_buffer_size`.

Using the configuration defaults, which feature an unbounded
`max_write_buffer_size`, should provide acceptable behavior.

Fixes #540

bin/propolis-server/src/lib/mock_server.rs

@@ -329,12 +329,7 @@ async fn instance_serial(
     query: Query<api::InstanceSerialConsoleStreamRequest>,
     websock: WebsocketConnection,
 ) -> dropshot::WebsocketChannelResult {
-    let config = WebSocketConfig {
-        // tune the buffer size limits down (compared to the defaults)
-        write_buffer_size: 4096,
-        max_write_buffer_size: 8192,
-        ..Default::default()
-    };
+    let config = WebSocketConfig::default();
     let mut ws_stream = WebSocketStream::from_raw_socket(
         websock.into_inner(),
         Role::Server,

bin/propolis-server/src/lib/server.rs

@@ -831,13 +831,13 @@ async fn instance_serial(
     let vm = ctx.vm().await?;
     let serial = vm.com1().clone();
 
-    let config = WebSocketConfig {
-        // tune the buffer size limits down (compared to the defaults)
-        // TODO: tuning for this could be explored
-        write_buffer_size: 4096,
-        max_write_buffer_size: 8192,
-        ..Default::default()
-    };
+    // Use the default buffering paramters for the websocket configuration
+    //
+    // Because messages are written with [`StreamExt::send`], the buffer on the
+    // websocket is flushed for every message, preventing both unecessary delays
+    // of messages and the potential for the buffer to grow without bound.
+    let config = WebSocketConfig::default();
+
     let mut ws_stream = WebSocketStream::from_raw_socket(
         websock.into_inner(),
         Role::Server,